SELECTION AND OPTIMIZATION OF SYNTHETIC FLOCCULANT FOR HIGH RATE DECANTERS (DEEP CONE THICKENERS)

Transcription

1 SELECTION AND OPTIMIZATION OF SYNTHETIC FLOCCULANT FOR HIGH RATE DECANTERS (DEEP CONE THICKENERS) Raghavan PKN 2*, Bamrah H 1, Kshatriya NK 2 and Dasgupta S 1 1 Alumina Refinery, 2 Research & Development and 3 Head Plant1 Bharat Aluminium co. Ltd., (A Unit of Vedanta Resources plc,) BALCo Nagar, Korba , Chhatisgarh State, INDIA Abstract Synthetic flocculants have come to play a prominent role in improving the operation of various systems in Alumina Refinery. They are used for improving settling and filtration characteristics of mud in settlers, washers and thickeners prior to disposal, and also to improve overflow liquor clarity and mud compaction. This paper describes the results of the Laboratory scale trials of various synthetic flocculants and establishes the best Flocculant for the Deep Cone Thickener (DCT). Preliminary screening tests were conducted for selection of the flocculants that gave best results with respect to good quality flocs, rapid settling rates (in the range of 115m/h) and better mud compaction. The screened flocculants from reputable suppliers were then assessed for suitability under plant conditions. The flocculant trails have shown that the flocculants play a crucial role in the mud dewatering. Flocculated red mud was observed to have better filterability, lower cake moistures and lower solid losses in filtrate with appreciable reduction in soda loss through the mud. A comparison of the experiments for a similar trial done for a conventional settler is also given. 1. Introduction In the Bayer process, flocculants are used to aid the separation of the red mud from the liquor, as well as to maximize the recovery of alumina and soda from the red mud in the washer circuit. The use of synthetic flocculants can also help in enhancing the production rates. Acrylamide sodium acrylate and hydroxymated polymers have gained importance as flocculants in the mineral processing industry. They have largely replaced the historically used flocculant like starch. The major advantage of these synthetic flocculants over natural ones is they are cheap, water soluble and can usually be tailored (composition and Molecular weights) to meet the specific applications The most recent type of hydroxymated flocculants (Hx) can accommodate a variety of process conditions. They vary in the degree of functionalization and molecular weight, but generally the removal of suspended solids increase molecular weight. However, higher settling rates are achieved at lower Hx product numbers, therefore, a compromise molecular weight is needed to optimise both settling rates and overflow clarity. The hydroxymated polyacrylamide polymers are available in the emulsion form and offer the following advantages: they replace both the polyacrylate and starch to provide exceptional clarities with high settling rates, better underflow densities, more consistent operation of the settlers / DCTs, improved filtration rates of the overflow liquor. The inclusion of the hydroxymic acid functional group in the new generation polymers provides a very strong attachment of the polymer to the mud particles. This strong attachment makes these materials significantly more efficient than the polyacrylates in capturing the fine particles and also generates large flocs which are more resistant to breakage. 1.1 Role of Flocculant in Red Mud Settling Polyacrylate containing carboxylic groups or acrylate / acrylamide based copolymers induce very high rate of settling and mud compaction. However, flocculants of this type cannot produce desired liquor clarity, making the use of starch required as well. Hydroximated flocculants containing carboxylic and amide groups simultaneously induce good settling rates and improve liquor clarity, but require high dosing rate. Hydroxamated flocculants also have positive impacts on liquor filterability, mud rheology, thickener life and lime consumption. In the recent years, many plants have adopted their use in combination with a traditional polyacrylate polymer (Jean, 4). The exact type of floc will depend on the refinery and the bauxite source (Kini, ). For example, in India, there are four varieties of bauxite, viz; (i) East Coast Bauxite which is highly Gibbsitic and soluble at low temperatures and atmospheric pressure, (ii) West Coast Bauxite with low Boehmitic and Diasporic content which can be digested at C at 5atm, (iii) Central Indian Bauxite which is Boehmitic which needs high temperature and pressure, (iv) Gujrat Bauxite being Diasporic in nature, requiring very high temperatures and pressures (32 C). The flocculants required for each system are different. 1.2 Factors influencing Flocculant type in Deep Cone Thickeners The flocculant activity of red mud and its settling behaviour is influenced by: the polymer molecular weight, the polymer composition, mode of application, effect of alkalinity. The most common method of disposal of Bayer red mud generated is by pumping the material to large settling ponds where waste disposal is permissible. In general, the rheological behaviour of mud residue is not only affected by solids loading, particle morphology and particle size distribution, but also by particle interactions (Frost, 7). These interactions are influenced by chemical factors such as presence of flocculants / deflocculant agents. High molecular weight anionic polyacrylates are commonly used for the flocculation of bauxite residue in DCT. For mud washing, a mixture of acrylate and nonionic acrylamide is often used. Most of the published literature on the flocculants for red mud settling is related to the behaviours of the material 64 Proceedings of the 8th International Alumina Quality Workshop 8

2 in conventional settlers, while very limited work exists on how the behaviour is affected in DCTs. The highsupersaturated aluminate liquor discharged from digestion must safely pass clarification before reaching precipitation. The main concerns in clarifications are; (i) reversion in the DCT and mud washers, (ii) scaling of polishing filter leaves, (iii) high soda losses through red mud, (iv) high hydrolysis loss, after passing through the sand separation. The digestion slurry is fed to DCTs for separation liquor and mud. The underflow passes through the mud washers where reversion can occur. The use of DCTs in combination with the washers reduces liquor to mud contact time and so a reductions reversion rates, and the soluble soda loss. Synthetic flocculants like polyacrylate along with starch were used in conventional settlers. Laboratory scale trials were conducted with an aim to get 115m/h settling rate and acceptable overflow clarity for the DCT. 1.3 Inadequate DCT sizing While changing from conventional settler operation to a DCT operation, it is almost impossible to prepare a representative DCT feed sample and design a DCT based on the necessary tests. As a result, there always exists a degree of uncertainty in getting DCT design. This can lead to a loss in production. Instead of purchasing new equipment, the use of suitable flocculants (alone or in combination) can often solve the problem. This paper describes the effects of various flocculants for settling of red mud generated from the digestion of Central Indian bauxite. An attempt has been made to compare the performance of flocculants for conventional settler with that for the Deep Cone Thickeners. 2. Experimental The bauxite was digested at high temperature (23524 C) and high pressure (335atm). This was carried out in a series of autoclaves with recuperative heating system. Slurry flashing is done in series of flash tanks. The flashed slurry is diluted and then sedimentation is carried out by flocculant. The diluted slurry from plant was collected for experimental work. 2.1 Flocculants Preparation A stock solution of 1% of hydroxamated flocculant was prepared in 1g/L NaOH in distilled water and diluted further to obtain a working solution of %. Working solution of % of polyacrylate was prepared in industrial water. 2.2 Settling Test for Optimum Dosage: Conventional jar settling tests in hot water baths was the first test used for selection of flocculant, optimization of concentration and doses regimes (single or split). The diluted slurry after desanding was poured into marked cylinders (1 Liter) and stored in a constant water bath (9798 C). The slurry was mixed and the hydroxamated flocculant was added in two portions with mixing (5:5). In the case of codosing, the polyacrylate flocculant was added first, the slurry was allowed to stabilize and then the hydroxamated flocculant was added. Fifty milliliters overflow was withdrawn, 15 minutes after flocculant was added and then filtered using a previously weighed suitable membrane filter paper for determination of overflow clarity. The compact volume was recorded after 3 minutes. The entire slurry was vacuum filtered to get total solid content. 3. Results and Discussion 3.1 Selection of Flocculants The results of laboratory tests evaluating the effectiveness of various flocculants were evaluated over a range of dosage levels. The best settling rates were obtained by hydroxamated floc 3 and polyacrylate A, relative to volumes dosed (178 and 64.9 respectively). It was observed that irrespective of the type of flocculant, the mud was highly sensitive to variations in dosage. The polyacrylate gave higher settling rate at higher dosage, but with poor overflow clarity. This trend reversed for hydroxamated flocculants (Figure 1) which showed poor settling rates but good clarity. The tests were targeted at ascertaining the order of magnitude of the dosage required to achieve a minimum settling rate of about 115m/h under the DCT conditions. The settling characteristics of the polyacrylate with different dosages are shown in Table 2. The settling rates vary from slurry to slurry even though the variations in the caustic concentration and the feed solids are minimal. This may be due to the changes in mud rheology at different caustic concentrations and feed solids. The effects of polyacrylate concentration and dosing method are given in Table 3, while the performance of hydroxamated flocculants in combination with polyacrylate is shown in Table 4. It was found that the flocculants which performed the best for conventional settlers did not meet the minimum criteria of settling rates required for the DCT. Based on laboratory tests, following can be summarized: (1) The required dosage of polyacrylate flocculant was lower than hydroxamated flocculant to achieve the same settling rates. (2) Depending on the specific flocculant used, a settling rate of 15m/h could be achieved if the hydroxamated flocculant was dosed at approximately (Table 1 and Figure 1). However, for the polyacrylate to achieve the same settling a dosing rate of 157g/T was required (Table 2 and Figure 2). (3) Table 3 shows that a concentration of.5% of polyacrylate gives better results than % concentration. Split addition and mixing of flocculants improve settling rate but not clarity. (4) Polyacrylate flocculant increased the settling rates but reduced overflow clarity, whereas this trend was reversed using hydroxamated flocculant (Figure 3 and Table 4). (5) There was no improvement in overflow clarity on increasing the ratio of hydroxamated flocculant in the combined flocculant doses and settling is better than the polyacrylate fraction is above 5% (Table 4 and Figure 4). (6) The hydroxamated / polyacrylate combination leads to a significant decrease in hydroxamated flocculant consumption (Table 5). (7) It was observed that the settling is very uneven in early stages, with no clear mud interface for polyacrylate. However, hydroxamated flocculant required no induction time and gave a very sharp interface from the beginning of settling with very clear liquor above the interface. 4. Conclusions From this work, the following overall conclusions can be observed: (1) The dosages of hydroxamated flocculant required to achieve a settling rate of 15m/h settling was in the range of with corresponding overflow clarities of mg/L. (2) The dosage of polyacrylate flocculant required to achieve a settling rate of 15m/h was in the range of 157 with corresponding overflow clarities of mg/l. Proceedings of the 8th International Alumina Quality Workshop 8 65

3 (3) The dosages of combined polyacrylate / hydroxamated flocculant required to achieve 15m/h settling rate was approximately (polyacrylate 66.3g + hydroxamated 58g and overflow clarity was 1mg/L). These results indicate that for identical settling rates, the flocculant mixture (hydroxamated and polyacrylate) provides suitable overflow clarity with lower overall dosages volumes, than if either flocculant was used individually. In the pilot plant scale trials conducted with a flocculant combination of 66.3gm of polyacrylate and 58gm of hydroxamated for a settling rate of 15m/h, we could observe an overflow clarity in the range of 1mg/L without starch addition. Filterability of the overflow liquor was enhanced which resulted in reduction in the filtration time of the aluminate liquor. Further, the mud compaction was also better due to the change in mud rheology due to the addition of the mixed flocculants. The soluble soda loss calculated for the filtered red mud was found to be 11kg/ T of alumina from 15kg/T of alumina (obtained in conventional settlers with mud raking mechanism where the flocculants used are to get a settling rate of 3 5m/h) with better flowability. It could also be observed that due to the change in the mud rheology, red mud filtration rate was enhanced. This is expected to give a substantial improvement in the Red Mud filtration in the Plant Scale. Table 1. Screening of Flocculants Flocculants Dosage g/t mud Hydroxamated Settling Rate m/h 6.6 Overflow solid mg/l 166 Table 2. Mud Settling Characteristics with Polyacrylate Type of Flocculants Feed Solid 59g/L, Na2O of Slurry 142.4g/L Flocculant Dosage g/t mud Settling Rate m/h Overflow solid mg/l Compaction ml Polyacrylate A 5.8 No Settling Hydroxamated Polyacrylate A Hydroxamated Polyacrylate A Polyacrylate A Polyacrylate B Table 3. Effect of Polyacrylate Flocculant concentration and dosing system Flocculant Dosage Flocculant con. % Dosing Settling Rate m/h Overflow solid mg/l Compaction ml Remarks Oc 147.1g/L Feed Solid 62.5g/L Oc 148.7g/L Feed Solid 78.3g/L Oc 148.7g/L Split(6:4) Feed Solid 78.3g/L 66 Proceedings of the 8th International Alumina Quality Workshop 8

4 Table 4. Mud Settling Characteristics of Hydroxamate with combination of Polyacrylate Flocculants Dosage Settling Rate m/h Overflow solid mg/l Compaction ml Remarks Hydroxamated Oc 149.2g/L Feed Solid 14g/L PA + Hydroxamated Hydroxamated Oc.8g/L Feed Solid 74g/L PA + Hydroxamated Hydroxamated Oc.4g/L Feed Solid 6.3g/L PA + Hydroxamated Table 5. Consumption of Hydroxamate and Polyacrylate combination Flocculants Dosage Settling Rate m/h Overflow solid mg/l Compaction ml Remarks Hydroxamated Hydroxamated Polyacrylate A + Hydroxamated 3 Polyacrylate A + Hydroxamated (133.2) (141.6) Oc.4g/L Feed Solid 6g/L PA Hydroxymated Trial 1 Trial 2 Trial Dosage (gm/t) Dosage (gm/t) Figure 1. Performance of Hydroxamated and Polyacrylate with different dosage Figure 2. Performance of Polyacrylate Flocculant Proceedings of the 8th International Alumina Quality Workshop 8 67

5 24 22 Hydroxymated Polyacrylate OF Clarity mg/ llt Figure 3. Effect of Settling rate on Overflow Clarity with Polyacrylate & Hydroxamate 1 17 Trial 1 OF Clarity mg/ llt Trial % Fraction of PA Dosage Figure 4. Effect of Polyacrylate fraction dosage on overflow clarity in a combination dose (Polyacrylate + Hydroxymate) Acknowledgements The authors are thankful to the management of Bharat Aluminium Company Limited, Korba, India, for granting permission for presenting this paper. The authors would like to thank Mr. Pramod Suri, CEO, Aluminium Business, Vedanta resources plc., India, Mr. Gunjan Gupta, CEO, BALCO Korba, India and Mr. Sharad Kumar Goel, Head, plant1, BALCO Korba, India for their encouragement during the course of this work. References Frost, Ray and Pejcinovic, Nik and Nguyen, Tai, Effect of surfactants on Bauxite residues suspensions, viscosity, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 292 (1), 7, pp Jean marc Rousseaux, Benoit Cristol, Stephane Torsiello, Combination of hydroxamated and Polyacrylamide based flocculants for settler performance improvements in ADG Alumina refinery, Light Metals, (4), pp 152. Kini, A.D, Misra, B.p., Raja, S.M., Effect of Synthetic Flocculants on Settling and Filtration of Red Mud, Symfloc (), India, pp 156. Lathouras, M., Avotins, P.V., The Role of Hydroxamated Polyacrylamide Flocculants in the Bayer process, Symfloc, (), India, pp Proceedings of the 8th International Alumina Quality Workshop 8